1. Filed of the Invention
The present invention relates to a control system in a hydraulic construction machine such as a hydraulic excavator or a bulldozer.
2. Description of the Related Art
Generally, a hydraulic excavator or a bulldozer is provided with a hydraulic pump which is driven by a prime mover. Hydraulic oil discharged by operation of the hydraulic pump is fed to actuators (for example a hydraulic cylinder) through control valves, which actuators actuate a working attachment such as an arm.
FIG. 3 shows an example of a conventional control system used in a hydraulic construction machine. A hydraulic pump 2 and a pilot pump 3 are connected to a shaft 10 of a prime mover 1. A control valve 5 is disposed between the hydraulic pump 2 and an actuator 4. Pilot oil pipes 53 and 54 are connected to a discharge oil pipe 31 of the pilot pump 3 through remote control valves 51 and 52. To the discharge oil pipe 31 is also connected a tank 6 through a relief valve 61. In accordance with operation of the remote control valves 51 and 52, hydraulic oil from the pilot pump 3 is fed to a pilot port of the control valve 5 to change over the state of the control valve 5.
In the above conventional control system, when the prime mover 1 is rotating at high speed and the actuator 4 is not operated (off-load), the pilot pump 3 is driven like an operating state (on-load) of the actuator, so that the hydraulic oil from the pilot pump 3 is not fed to the pilot oil pumps 53 and 54 but is returned to the tank 6 wastefully through the relief valve 61. Consequently, there occurs a large power loss. Moreover, upon turning OFF of the prime mover 1, the pilot pump 3 also stops operation, with the result that the main oil pressure source runs out and at the same time the pilot oil pressure source also runs out, thus making it no longer possible to control the control valve 5.